A series of in vitro constructed recombinants of endogenous and exogenous avian leukosis viruses (ALVs) will be used in lines of chickens which have been bred for endogenous virus content to study the role of specific ALV sequences in tissue tropism, the effect of thymotropism on immune response, and immune response to avian leukosis virus infections. Studies on tissue tropism will focus on the role of the LTR, env, and sequences near the 5' LTR in the tissue specific replication of ALVs. Studies on immune response will evaluate the relative roles of internal and external viral proteins in eliciting protection against ALV and avian sarcoma virus infections. Immune response to retrovirus infections will also be approached using avian leukosis virus vectors to express human immunodeficiency virus (HIV) envelope and core (gag) proteins in immunocompetent chickens. B-cells responses to the expressed HIV proteins will be measured by assaying sera of inoculated birds for titers of neutralizing activity for HIV and titers of antibodies to specific HIV proteins. These data will be compared to similar data collected for immune responses to ALV env and gag proteins and to influenza virus H7 protein expressed by the same vector. The purpose of these studies is to define the immunogenic potential of specific HIV proteins in hosts whose immune system is not compromised by HIV infection and to compare the immunogenic potential of HIV proteins to comparably expressed ALV and flu proteins.